by Calum MacLeod, USA TODAY

by Calum MacLeod, USA TODAY

BEIJING - North Korea warned Friday it will take "strong physical countermeasures" against South Korea if Seoul takes part in United Nations sanctions aimed at punishing Pyongyang for the rocket launch.

"Sanctions mean war and a declaration of war against us," the Committee for Peaceful Unification of the Fatherland said in a statement carried by North Korea's official Korean Central News Agency.

The threat comes a day after North Korea vowed Thursday to target the USA, its "sworn enemy," with a nuclear test and further long-range missile tests, as Pyongyang continued to reject a U.N. Security Council resolution that expanded sanctions against the already highly isolated nation.

South Korea said Friday it will not tolerate North Korean provocations but will continue to push for dialogue with Pyongyang, a special envoy to President-elect Park Geun Hye said after the North's top governing body declared it would continue atomic tests and rocket launches.

"President-elect Park makes it clear that North Korea's nuclear ambitions and further provocations against the South will not be tolerated," envoy Rhee In Je said at the World Economic Forum in Davos, Switzerland.

The U.N. resolution Tuesday condemned the North's Dec. 12 rocket launch that itself breached earlier U.N. resolutions. North Korea claimed that launch was a peaceful satellite mission, but the U.S. and South Korean governments said it was a test of Pyongyang's banned ballistic missile program.

In a new phase of "our century-long struggle against the United States, we do not hide the fact that various satellites, long-range missiles that we will continue to launch and high-level nuclear test we will conduct will target our sworn enemy, the United States," said a statement Thursday from the National Defense Commission, the North's top military body, reported the state-run Korean Central News Agency.

Rejecting the possibility of dialogue about the denuclearization of the Korean Peninsula, including the long-stalled six-party talks hosted by China, the commission promised to "launch an all-out action to foil the hostile policy toward the DPRK being pursued by the US and those dishonest forces following the US."

Some North Korea watchers based in South Korea said Pyongyang's threats Thursday were expected and familiar, but one expert detected a new seriousness. Besides genuine acts of military aggression, the famously bellicose North has for decades waged a war of words with the "US imperialists" and their "puppets" in South Korea, where the USA stations nearly 30,000 troops.

"It's not a real surprise to see North Korea's harsh response," said Park Young Ho, a researcher at the Korea Institute for National Unification in Seoul. "And it's only slightly harsher than normal." North Korea, which already sees itself as a nuclear weapons state, will only join negotiations on denuclearizing the peninsula if the U.S. grants it equal status as a nuclear state, he said.

The language from the North was similar to previous outbursts from Pyongyang, agreed Tong Kim, an international relations expert at Korea University in Seoul. Last month's launch "was successful as it gave legitimacy to Kim Jung Un's power base, he carried out his father's will," he said. Kim succeeded his father Kim Jong Il in 2011, the third generation of a family dictatorship founded by grandfather Kim Il Sung.

The international community had to increase sanctions, but "sanctions have never worked on North Korea and I don't think they will again," Tong said. "We must somehow open up a window of dialogue, nothing will be solved with confrontation or crisis."

South Korean military investigators who examined debris from the December launch concluded that North Korea made most of the key parts itself. But the nation still has "a long way to go" before its intercontinental missile technology is advanced enough to worry U.S. authorities, Tong said. He doubted the North will soon conduct another nuclear test, due to a limited supply of plutonium, but expressed concern over its uranium enrichment program, another way to make atomic weapons.

"Whether North Korea tests or not, it's up to North Korea," said Glyn Davies, the U.S. special envoy on North Korea policy, in Seoul Thursday, reported the Yonhap news agency. "We hope they don't do it, we call on them not to do it. It will be a mistake and a missed opportunity if they were to do it," he said.

A third nuclear test, after previous underground tests in 2006 and 2009, "is very likely, in a few weeks, after the North sees how the U.S. and South Korean governments respond to their harsh statement," said Lim Eul Chul, a professor of North Korean studies at Kyungnam University in Seoul. "I never saw this kind of seriousness before" in the North's response, which reflects Pyongyang's disappointment that the USA has not pursued a more "positive" relationship, he said.

Besides the removal of sanctions, Pyongyang craves more attention from the USA after years of "strategic patience" under Obama, he said. The North's officials "hate that kind of words," Lim said. "Without dialogue, frustration will worsen," said Lim, who hopes South Korea's President-elect Park Geun Hye will help foster renewed exchanges with the North.

Even seasoned North Korea observers struggle to understand this "extraordinary and unique country," Park Young Ho said. "The leadership and ordinary North Korean people see the world in a very different way than outsiders," while the expanded sanctions this week will fit into their narrative of "continuing pressure and oppression by the outside world," he said.

The latest Kim to hold power may disappoint anyone hoping for substantial change, Park said. "I don't think Kim Jong Un is likely to become North Korea's Deng Xiaoping or Gorbachev," he said, referencing the leaders who transformed China and the Soviet Union respectively. "He's a new version of Kim Jong Il or Kim Il Sung."

U.S. Defense Secretary Leon Panetta said he has seen no outward sign that North Korea will follow through soon on its plan to conduct a test. But that doesn't mean preparations aren't taking place.

"They have the capability, frankly, to conduct these tests in a way that make it very difficult to determine whether or not they are doing it," Panetta said.

North Korea is estimated to have stored up enough weaponized plutonium for four to eight bombs, according to scientist Siegfried Hecker, who visited the North's Nyongbyon nuclear complex in 2010. In October, an unidentified spokesman at the National Defense Commission claimed that the U.S. mainland was within missile range. And at a military parade last April, North Korea showed off what appeared to be an intercontinental ballistic missile.

White House spokesman Jay Carney on Thursday said North Korea's aggressive stance is unnecessary and warned against any further testing.

"North Korea's statement is needlessly provocative and a test would be a significant violation of United Nations Security Council resolutions," he said.